One electric component measurement apparatus has been disclosed in Japanese Patent, Laid-Open No. 63373/1977, that was filed for thee present applicant. This measurement apparatus determines the resistance of a capacitor by converting current flowing through it ito a voltage and phase detecting the voltage output.Generally speaking, however, the resistance is far smaller than the capacitance. Therefore, if this resistance and capacitance are to be measured by the same circuit e.g., a phase detector, the accuracy of the measurement of the resistance cannot be improved in dependence upon the angular error of the phase detector. Moreover, since the phase detector has a limited dynamic range, the measurement cannot be highly accurate for a widely different values of capacitance.
One apparatus for measuring the resistance of a capacitor is shown in FIG. 1. This measurement apparatus uses a circuit 41 for cancelling or offsetting capacitance that is comprised of a variable voltage divider 42, a phase shifting capacitor 43 and an inverting amplifier 44 connected in series between a source 40 of a voltage V and the inverting input of an operational amplifier 46. A capacitor 45 having a loss resistance Gx to be measured is connected between the source 40 and the inverting input of the operational amplifier 46 so that a current (i.sub.CX =WCxV and i.sub.GX =V/Gx) that flows through the capacitor 45 is converted into a voltage by one of a plurality of range resistors 47 that is connected between the inverting input of the operational amplifier 46 and its output. The output is applied to a vector voltage ratio meter 49. The circuit 41 absorbs or cancels some of the current flowing through the capcity Cx of the capacitor 45. The resistance Gx is obtained by the phase detection in the vector voltage ratio meter 49, and the capacitance Cx is determined by adding the capacitance obtained by the phase detection of the vector voltage ratio meter 49 and the set value resulting from the setting of the variable voltage divider 42 in the offset circuit 41.
In case, however, the value of the capacitor varies over a considerable range, e.g., 1 pF to 2,048 pF, the measurement apparatus has found it difficult to absorb or cancel the current component i.sub.CX throughout that range, especially where the capacitance and the current component i.sub.CX are high, so that an unabsorbed or uncancelled component of i.sub.CX flows through the range resistor 47. Although the resistance of the range resistor 47 can be varied in accordance with the magnitude of the current component i.sub.CX, the resolution of the offset circuit 41 is a 1 pF step within a large capacitance range, e.g, 1 to 2,048 pf so as to make it difficult to absorb or cancel the current component i.sub.CX with the same resolution for different magnitudes of the capacitance Cx. This is because the variable voltage divider 42 in the offset circuit 41 is a transformer, as will be described hereinafter, thereby making it difficult to obtain step exceeding 1pF.
This makes it difficult for this measurement apparatus to expand and transmit the loss angle to the vector voltage ratio meter equally over the whole measurement capacitance range so that the measurement apparatus has failed to measure the loss with accuracy.
FIG. 2 is a circuit diagram showing the variable voltage divider 42 in the capacitance offset or cancellation circuit 41 shown in FIG. 4. A transformer 51 has a number of secondary windings, with respective turns ratios of (1/2).sup.n so as to generate a desired voltage V.sub.0 in accordance with the connected position or the programmed value, of one of n switches, only four of which, 52-56 are shown. In order to generate a voltage of 1/2V, for example, only the switch 53 is connected at its side a whereas the remaining switches are connected at their sides b.
In this variable voltage divider 42, however, the characteristics of the transformer 51 a viewed from the input side change with the switch that is closed so as to degrade the linearity between the switch closed and the voltage dividing ratio. Therefore, a correction impedance 57 is used. This complicates the circuit structure. Another defect is that the resolution (1/2).sup.n cannot be greatly increased.